1. Field of the Invention
The present invention relates generally to adhesives used to bond electronic components to circuit boards. More particularly, the present invention relates to flexible epoxy based compounds which are designed to provide releasable bonding of the electronic components to circuit boards.
2. Description of Related Art
The design and fabrication of electronics typically involves bonding the electronic components to circuit boards using glass cloth or other suitable fiber cloth impregnated with a "flat pack" adhesive. The term "flat pack" is used herein to mean an electronic component or module having at least one flat or planar surface.
There are a number of specific characteristics which flat pack adhesives must have in order to provide adequate bonding of electronic components. The flat pack adhesive should have high thermal conductivity, on the order of 0.3 (BTU) (Ft)/(Ft.sup.2) (hour) (.degree.F.), in order to remove heat generated by the electronic component. These high levels of thermal conductivity are typically achieved by loading the adhesive with relatively high amounts of fillers such as alumina and other heat conductive materials. Accordingly, the flat pack adhesive must be capable of retaining its adhesive qualities even at the high filler loadings required to achieve desired rates of thermal conductivity.
The flat pack adhesive should have dielectric properties which include volume resistivity, dielectric strength, dissipation factor and dielectric constant comparable to good insulators. Lap shear strengths on the order of 100 psi to 500 psi are also necessary. In addition, for use in space applications, the adhesive must meet National Aeronautics and Space Administration (NASA) outgassing standards. Furthermore, the flat pack adhesive must be thermally stable to provide sufficient flat pack bonding throughout the range of temperatures to which spacecraft electronic components are subjected.
In addition to the above physical and chemical requirements, the flat pack adhesive must also meet certain criteria in order for it to be commercially acceptable. For example, the adhesive formulation must be capable of being stored as a premix material which can be ready for use at any time. Preferably, the adhesive formulation should be capable of being stored as a frozen premix at temperatures of -40.degree. F. for periods of at least three months. Moreover, the adhesive should have a relatively short cure time at reasonably low temperatures. For example, cure times on the order of 20 minutes at temperatures of up to 200.degree. F. are desirable. Further, when used in the form of a film adhesive, it is sometimes advantageous that the viscosity and handling characteristics of the material are such that it can be easily, reliably and reproducibly impregnated into the various fiber cloths for final use in bonding the electronic components to circuit boards.
An additional requirement of the flat pack adhesive is the ability to function as a releasable adhesive It is very desirable in many electronic systems that the ability be provided to remove individual components from the circuit board. This allows removal and replacement of individual defective or obsolete components without having to replace the entire circuit board. Accordingly, it is important that the flat pack adhesive be sufficiently strong to prevent the electronic components from vibrating loose, while still allowing removal of the component when desired without damage to the component or the circuit board. The term "releasable bond" as used herein is intended to mean a bond which holds two surfaces together but which may be broken, if desired, without damaging either surface.
Currently, flat pack bonding is accomplished with traditionally used polysulfide adhesives. However, polysulfides have been shown to possess poor thermal stability in a vacuum (i.e. space environment) which results in unacceptable outgassing. Recently, polyurethane adhesives have been used for flat pack bonding. The polyurethane adhesives meet many of the above-described requirements for flat pack bonding. However, polyurethane does not work well as a frozen film adhesive due to condensation of water upon thawing and subsequent reaction of water with the diisocyanate component of the adhesive generating carbon dioxide gas. The gas forms permanent voids and bubbles in the cured adhesive.
Flexible polyglycol based epoxy formulations have not previously been used as flat pack adhesives. Flexible epoxy resins are generally not recommended by their manufacturers for use as the main constituent for general applications due to their inherent weakness.
There presently is a continuing need to develop flat pack adhesives which meet all of the desirable physical, chemical and commercial requirements necessary to provide releasable bonding of electronic components to circuit boards in general, and, more specifically, for spacecraft applications.